A highly experienced FBI investigator and former army officer hired by special counsel Robert Mueller to examine Russia's interference in the 2016 election has unexpectedly stepped away from the probe, ABC News reported on Wednesday.

Peter Strzok, a veteran counterintelligence investigator, is now working for the FBI's human resources division, according to ABC. It is unclear why he stepped aside, or if he did so voluntarily.

Asha Rangappa, a former FBI counterintelligence agent and associate dean at Yale Law School, said that she had "never heard of an agent being moved to the human resources department."

"I have seen instances where if some issue comes up, the agent might be moved to another investigation or to the operations center, where you essentially field calls all day," Rangappa said. "But why he would be moved to HR is just bizarre."

Rangappa did not want to speculate on what may have happened in Strzok's case, but said there were many factors — ranging from small administrative violations to more significant incidents — that could raise questions about an agent's ability to stay on a case.

A former FBI agent who worked with Strzok on and off over several years in the bureau's counterintelligence division said that Strzok's move to HR means he has now been separated from counterintelligence work altogether.

The FBI sometimes parks agents in the human resources department, the agent explained, when they need to be reassigned quickly away from substantive matters and there's no other place to put them. Christopher Wray, who was confirmed as the new FBI director two weeks ago, would have played a role in reassigning Strzok.

Strzok headed the FBI's counterespionage division last year and was one of the top officials overseeing the criminal investigation into whether Hillary Clinton mishandled classified information while she was secretary of state. He had previously worked on some of the "most secretive investigations in recent years involving Russian and Chinese espionage," according to the New York Times.

Rangappa noted that the DOJ's Office of the Inspector General (OIG) opened an investigation in January into the FBI's handling of the email probe, including former FBI Director James Comey's decision to announce a new inquiry into her email server 11 days before the election. It is not clear whether Strzok, who supervised elements of the email probe, was caught up in the OIG investigation.

The OIG declined to comment. But their website lists the probe as ongoing.

Strzok's departure also came one week after The Washington Post reported that Mueller had obtained a search warrant to raid the home of President Donald Trump's former campaign chairman, Paul Manafort. The Post report cited "people familiar with the search," prompting questions about whether anyone on Mueller's team had leaked the existence of the search warrant to the Post.

Mueller has assembled two-dozen investigators and lawyers to help him examine Russia's election interference and whether Trump's campaign colluded with Moscow to undermine Clinton. The former FBI director impaneled a grand jury in late July that quickly issued subpoenas related to the June 2016 meeting between Trump's eldest son and a Russian lawyer with connections to the Kremlin.

Manafort and Trump's son-in-law and senior adviser, Jared Kushner, also attended the meeting.